The present invention relates to a magneto-optical recording medium which allows direct overwriting by light modulation and reproduction with superresolution (hereinafter referred to as "superresolutive reproduction"), and to a recording/reproduction method for this recording medium.
There have heretofore been known, as magneto-optical recording media, various magnetic films of rare earth-transition metal alloys, such as TbFeCo film. However, these conventional magneto-optical recording media involve problems such as a limited recording density because it is impossible to reproduce information recorded in a bit domain which is smaller than the optical resolving power of an optical system for reproduction, a low recording speed because direct overwriting is impossible, i.e., recording must be performed after erasure of previously recorded information.
There have recently been proposed a method of superresolutive reproduction to address the former problem and an exchange-coupled four-layers film allowing direct overwriting to address the latter.
The method of superresolutive reproduction is described in, for example, Japanese Journal of Applied Physics, Vol. 31, Part 1, No. 2B, February 1992, pp. 56814 575. FIG. 7 is an explanatory view showing the layered structure of an magneto-optical recording medium allowing superresolutive reproduction, with indication of the magnetization direction of each layer by an arrow. The operation in the method of superresolutive reproduction will be described with reference to FIG. 7 wherein numeral 11 denotes a reproduction layer, numeral 12 denotes a switching layer, numeral 13 denotes a memory layer, numeral 14 denotes a mask region, numeral 15 denotes a light spot, numeral 16 denotes a record bit domain, and numeral 17 denotes an unrecorded region. The magnetization of the reproduction layer 11 is aligned in the same direction as that of the memory layer 13 at room temperature by exchange-coupling through the switching layer 12. When the magnetic layer is given the energy of reproduction light, a temperature distribution is produced of which peak appears on the medium-advancing side of the reproduction light spot. Within such a temperature distribution of the magnetic layer the portion of the switching layer 12 which is heated above the Curie temperature thereof cuts off the exchange-coupling between the reproduction layer 11 and the memory layer 13. Hence, the magnetization direction of the reproduction layer 11 at the region coincident with that portion of the switching layer 12 is no longer restrained by the memory layer 13 and is, therefore, aligned with the direction of an external magnetic field so as to be identical with each other. At this time that region of the reproduction layer 11 within the reproduction light spot, of which magnetization direction is aligned with the direction of the external magnetic-field, becomes a mask region 14, which will not contribute to a reproductive signal component. Therefore, the reproductive signal is detected from the region other than the "mask" region. This means that the diameter of the light spot is virtually reduced. In other words, it is possible to achieve reproduction from a minute magnetic bit domain which is beyond the limit of an optical resolving power ruled by the diameter of a light spot. That is, superresolutive reproduction is feasible. To form the "mask" required for the superresolutive reproduction, a reproduction light beam needs to have a certain degree of intensity. The intensity of a reproduction light beam for the superresolutive reproduction is represented by P.sub.R hereinbelow.
On the other hand, the exchange-coupled four layer film allowing direct overwriting is described in, for example, Japanese Journal of Applied Magnetics, Vol. 14, No. 2, 1990, pp. 165 to 170. FIG. 8 is an explanatory view for illustrating a direct overwriting operation of the aforesaid four layer magneto-optical memory medium allowing direct overwriting based only on modulation of light intensity. In FIG. 8, a large arrow indicates the magnetization direction of each layer, while a small arrow in the large arrow the magnetization direction of the transition metal sub-lattice of each layer. The four layer magnetic film includes, from the top, a memory layer 21, recording layer 22, switching layer 23 and an initializing layer 24. The Curie temperatures of the layers 21 to 24 are represented by T.sub.c1, T.sub.c2, T.sub.c3 and T.sub.c4, respectively. T.sub.room represents room temperature, T.sub.comp2 the compensation temperature of the recording layer 22, and T.sub.comp4 the compensation temperature of the initializing layer 24. The direct overwriting operation will be described in the order of (A) initializing operation, (B) high-temperature operation and (C) low-temperature operation.
In the initializing operation (A), after the formation of the magneto-optical recording medium, the respective transition metal sub-lattice magnetization directions of the recording layer 22, switching layer 23 and initializing layer 24 are made to align in the same direction, for example, in the downward direction in the drawing (refer to states (a) and (b) in FIG. 8). This is achieved by applying a sufficiently large magnetic field first. At this time, the transition metal sub-lattice magnetization direction of the memory layer 21 may be aligned in either the upward (refer to state (a) in FIG. 8) or downward direction (refer to state (b) in FIG. 8).
In the high-temperature operation (B), the magnetic film is irradiated with a recording light beam of a high intensity thereby to raise the temperature thereof to a temperature in the vicinity of the Curie temperature of the recording layer 22. Then magnetization of the memory layer 21 and switching layer 23 is lost and the magnetization direction of the recording layer 22 is aligned in the direction (the upward direction in the drawing) of an external magnetic field (refer to state (c) in FIG. 8), regardless of the initial state, (a) or (b) in FIG. 8. In a cooling step that follows, when the temperature of the magnetic film drops to a temperature below the Curie temperature of the memory layer 21 and the magnetization of the memory layer appears, the transition metal sub-lattice magnetization direction of the memory layer 21 is aligned with that (the upward direction) of the recording layer 22 by an exchange-coupling force (refer to state (d) in FIG. 8). Further, when the temperature of the magnetic film drops to a temperature below the Curie temperature of the switching layer 23 and the magnetization of the switching layer appears, the transition metal sub-lattice magnetization direction of the recording layer 22 is aligned with that (the downward direction) of the initializing layer 24 through the switching layer 23. Thus, the state (a) in FIG. 8 is restored. In the above procedure the transition metal sub-lattice magnetization direction of the initializing layer 24 is set to always align in one direction.
In the low-temperature operation (C), the magnetic film is irradiated with recording light beam of a low intensity thereby to raise the temperature thereof to a temperature in the vicinity of the Curie temperature of the memory layer 21. Then the transition metal sub-lattice magnetization direction of the memory layer 21 is aligned with that (the downward direction) of the recording layer 22 (refer to state (e) in FIG. 8) by an exchange-coupling force, regardless of the initial state, (a) or (b) in FIG. 8. When the magnetic film is cooled to room temperature, the state (b) in FIG. 8 is resumed.
Since the magnetization direction of the memory layer 21 is aligned in the upward direction by the high-temperature operation (B) or in the downword direction by the low-temperature operation, direct overwriting can be achieved if the intensity of recording light beam is modulated in a binary fashion, i.e., high or low in accordance with binary-coded information "0" or "1". Hereinafter the high-intensity of the recording light beam for the high-temperature operation (B) will be represented by P.sub.H, while the low-intensity thereof for the low-temperature operation will be will be represented by P.sub.L.
As described above, there have been proposed, on one side, a magneto-optical recording medium capable of superresolutive reproduction and, on the other side, one allowing direct overwriting. However either the former or the latter does not allow both superresolutive reproduction and direct overwriting. To make these merits compatible with each other in one magneto-optical recording medium, the light beam needs to have three degrees of intensity, i.e., a light beam intensity P.sub.R for the superresolutive reproduction in addition to the two light beam intensities for overwriting, P.sub.H for the high-temperature operation and P.sub.L for the low-temperature operation. Further, it is desired that an expected operation be assuredly achieved in accordance with each light beam intensity so as to make satisfactory superresolution behavior and satisfactory overwriting behavior compatible with each other, and that the light beam intensities P.sub.H, P.sub.L and P.sub.R each have a sufficient margin (or allowance). A phenomenon must not occur such that during superresolutive reproduction at the light beam intensity P.sub.R the low-temperature operation happens thereby changing the recorded information. In addition, where a magneto-optical material, such as a NdFeCo film, Pt/Co multilayered film or the like, which produces a large reproductive signal output in response to light of a short wavelength is used in the medium allowing both superresolutive reproduction and overwriting, it is not clarified yet how and what to do in order to improve both superresolution behavior and overwriting behavior as well as the behavior in response the light of a short wavelength.
Either is not clarified yet the relation between the direction of an external magnetic field to be applied for overwriting and the direction of an external magnetic field to be applied for superresolutive reproduction when direct-overwritten information is to be reproduced.
The present invention has been attained to overcome the foregoing problems. It is, therefore, an object of the present invention to provide a magneto-optical recording medium which allows superresolutive reproduction and direct overwriting.
It is another object of the present invention to provide a recording/reproduction method which is capable of recording or reproducing information without destroying recorded information within an extensive light intensity range of reproduction light beam with use of a magneto-optical recording medium allowing superresolutive reproduction and overwriting.